A tap changer for interruption-free changing under load, e.g. the selection of taps on, for example, a power transformer may be used for voltage regulation in power distribution and generating systems. In such arrangements, a switchover impedance is provided which is effective during tap change operations to avoid short circuiting and overloading.
A tap changer of the type with which the invention is concerned is the load tap changer type RMV II of the firm Reinhausen Manufacturing, Humboldt, USA, and described in the brochure RM 05/91-1094/5000. A vacuum switching cell is here used for the switching under load.
More particularly, the system can comprise a pair of selector switches which are movable from tap to tap along the row of transformer taps of a load transformer. Each one of these switchover contacts is connected to a respective impedance, e.g. a coil. The opposite ends of the coils can be bridged by a vacuum switching cell and each of these impedances is also connected at the aforementioned opposite end with a fixed contact of a bypass switching arrangement. In addition to the fixed bypass contacts, the bypass switching system includes movable contacts which can be selectively operated so that both switches may be closed or each movable bypass contact may be in an open-circuit position while the other is closed. Depending upon the setting of the bypass switch, therefore, each of the two movable bypass contacts can be individually connected with the load line or conductor or both of the movable contacts can be connected in common to the load line or can connect the load line in common to their respective fixed contacts. For this purpose, the system has required two fixed bypass contacts and two movable bypass contacts, the movable bypass contacts being linked together and with the load conductor.
In a stationary state, the bypass switch connects the load line L in common with the two fixed bypass contacts. At the beginning of a tap change under load, this connection between the two fixed bypass contacts via the movable bypass contacts is interrupted so that, for example, one of the bypass contacts is opened when the vacuum cell switch is closed for the beginning of switchover. The bypass switch thus does not have to break under load. The vacuum switching cell can then open circuit, whereupon a selector contact can be moved from one tap to the other whereupon the vacuum switching cell is then closed and the bypass contact can then close. The other bypass contact can then open and the process repeated until the second selector contact is on the successor tap. The bypass switch with this functioning is described, for example, in German patent 40 11 019.
A bypass switch system of this type has numerous drawbacks. The link mechanism which connects the bypass contact electrically as well as mechanically and provides their coupled movement is complex and expensive to fabricate. The same applies for the fixed bypass contacts which must be held in a frame and are constituted by individual lamella. If the current capacity is to be high, both the fixed and movable contacts are comparatively large which is also undesirable.